The present application hereby claims priority under 35 U.S.C. xc2xa7 119 on German patent application number DE 10219388.6 filed Apr. 30, 2002, the entire contents of which are hereby incorporated herein by reference.
1. Field of the Invention
The invention generally relates to a method for producing a trench structure in the surface of a polymer substrate, preferably by irradiation with a laser of a predetermined wavelength.
2. Background of the Invention
WO 00/16443 discloses the production of hole and trench structures in printed circuit boards, in which case such trench structures may serve for example for forming shielded conductor structures in the printed circuit boards. Besides plasma etching, the use of a laser is also mentioned generally therein as a device for producing such trenches in a printed circuit board.
However, when a laser radiation is employed to remove polymer material, one problem results from the fact that the energy distribution within a laser beam is not uniform, but rather follows a Gaussian distribution. Thus, a lower energy density is emitted onto a substrate in the edge regions of the laser beam than in its center. The consequence of this is that, during the fabrication of a trench structure, for instance, the sidewalls of the trench cannot be produced with the desired steepness. An additional problem arises in the case of substrates in which the polymer material is reinforced with glass fibers or the like, this reinforcement material requiring a higher energy density for removal than the polymer material itself. This has the effect that, in the edge regions of a trench fabricated in this way, the glass fibers are in part only melted but not removed and adhere to the walls in drop or ball form. Since such small glass balls cannot be removed economically either mechanically or by chemical etching, they constitute a serious problem since they disrupt the surface of the trench walls and impede a subsequent coating.
Therefore, it is an aim of an embodiment of the present invention to specify a method by which it is possible to obtain trench structures with clean sidewalls and an acceptable steepness in polymer substrates, in particular in those with glass fiber reinforcement.
According to an embodiment of the invention, this may be achieved by the following method steps:
a conformal mask made of a material which reflects the laser radiation is arranged on the surface of the substrate, which mask has cutouts corresponding to the trench structure to be produced, and
the laser beam is then guided over the cutouts of the mask, said laser beam being guided at least in each case once over the boundary edge of the respective cutout in an overlapping manner such that the energy density of that proportion of the laser beam which impinges on the polymer surface, at each point, lies above a threshold at which the substrate material is completely removed.
Thus, in the case of the method according to an embodiment of the invention, with the aid of a conformal mask in each case in the edge region of the trench to be produced, that proportion of the laser beam is cropped, i.e. reflected by the mask, which would in any event not suffice to completely remove the substrate material.
An embodiment of the invention can be employed particularly advantageously when producing trench structures in a glass-fiber-reinforced substrate, the mask then shielding that edge region of the laser beam which lies below the energy threshold which is necessary for vaporizing the glass fiber material and is achieved for example in the case of a peak power density of about 1-10 MW/cm2, preferably 6-7 MW/cm2, depending on the material.
Preferably, a laser whose wavelength is reflected to a great extent at the mask layer, preferably comprising copper or a copper alloy, is used for producing the trench structure. In this case, the laser beam can be focused directly onto the surface of the substrate. Lasers having a wavelength of 9 xcexcm to 11 xcexcm are preferably used, in particular a Q-switched CO2 laser having a pulse frequency of 10 to 200 kHz, preferably of approximately 100 kHz, and a pulse duration of 50 to 500 ns, preferably approximately 150 ns. However, it is also possible to use a radio-frequency-excited (RF-excited) pulsed CO2 laser having a pulse frequency of between 1 and 15 kHz, preferably between 3 and 5 kHz, and a pulse duration of between 1 and 20 xcexcs, preferably between 3 and 5 xcexcs.
In another advantageous refinement, it is also possible to use a TEA-CO2 laser (transversely excited atmospheric laser) having a pulse frequency of between 1 and 15 kHz and a pulse duration of between 50 and 100 ns, preferably about 70 ns.
Instead of the CO2 laser, however, it is also possible to use other lasers. It would even be possible also to use a UV laser if such a laser is available with a sufficient power and acceptable operating speed. However, since such a UV laser, for example, is reflected only to a small extent by a mask layer made of copper, it is not permitted to be focused directly onto the plane of the mask or the substrate surface.
The mask used in the case of an embodiment of the invention is preferably a metal layer which is applied to the surface of the substrate and, as mentioned, preferably comprises copper or a copper alloy. The metal layer may be formed by chemical deposition or electrodeposition on the surface of the substrate, the cutouts then being produced by partial removal of this metal layer by chemical etching methods or mechanical separation methods.
In a particularly advantageous refinement, the mask is also formed by a patterning of the metal layer by use of a laser, a different laser being used for this than the one used for producing the trench structure in the substrate. Therefore, a laser whose wavelength is readily absorbed by the material of the mask if preferably used here; an appropriate laser for this is one having a wavelength of between 250 and 1100 nm, for example a UV laser having a wavelength of 266 or 355 nm. A solid-state laser pumped with diodes or with a flashlight lamp and having subsequent frequency multiplication may be involved in this case, which laser is operated with a pulse frequency of above 1 kHz up to 200 kHz and with a pulse duration of between 1 ns and 200 ns, preferably between 10 and 60 ns.
Depending on the width of the trenches to be produced in the substrate and depending on the spot width of the laser beam, either a single pass with the laser over the mask cutout may suffice for producing the trench structure, or the laser beam is guided in a plurality of tracks lying next to one another through the cutouts of the mask. These laser processing tracks may run in the longitudinal direction of the trenches or else be guided in meandering fashion transversely with respect to the longitudinal extent of the trenches.
The mask used according to an embodiment of the invention makes it possible, moreover, to realize a variable ratio of depth to width of the removed trenches. Generally, a ratio (aspect ratio) of 1:1 (depth=width) is expedient, but other ratios are also possible, for example 1:3 or more or 3:1 or more, in other words a wide and shallow or a narrow and deep trench being realized.